The invention relates to the manufacture of flexible disks or records which can be optically read by concentrated radiation and more specifically to a method for the manufacture of such disks and to an apparatus for performing this method.
Optically legible disks are generally such that the information is recorded on a track in the form of micro-depressions, whose width is approximately 1 micrometer and length is variable. This track can be in the form of concentric circles or can be spiral track, the pitch between the grooves in the radial direction being between 1.5 and 2 .mu.m. Such disks make it possible to record a large quantity of optically legible data. Reading is obtained by diffraction of radiation concentrated on the plane containing the data when the disk is rotating, the reading device having vertical and radial slaving devices making it possible to maintain the focusing point of the reading beam on the track. Reading can be carried out by transmission or by reflection. In the case of reading by transmission, a disk recorded on its two sides can be read without being turned over by merely moving the focusing plane of the reading beam. When the track is a spiral track and if the spirals are in opposite directions on the two sides, the disk can be read in a continuous manner, without interruption for passing from one side to the other. Thus, the reading of the first side is carried out by following a given radial progression direction and then at the end of the track the focusing plane of the beam is brought onto the second side, without any radial displacement of the beam head and the second side is then read by following the radial progression direction, which is opposite to that of the first side.
Thus, such a reading device by transmission can permit a continuous reading of a programme which can take up to 1 hour and a random access to one of the sides, by merely modifying the focusing plane of the reading beam.
However, for such a device to function in a completely satisfactory manner, it is absolutely necessary, taking account of the dynamics of the radial control and of the density of the recorded information, that centering defects of the disk with respect to the reading device and consequently the off-centering of the central hole of the disk with respect to the track does not exceed about 20 microns. Thus, the manufacturing process by pressing such disks must be such that the eccentricity faults remain within this range.
In the manufacturing methods used for flexible audio disks, the operation of drilling the central hole can be performed by punching the disk, still located on the pressing means following the pressure-temperature cycle. In the case of rigid audio disks obtained by injection moulding or thermal compression, the material can be injection moulded or thermally compressed around a centering means, the central hole being obtained by moulding during the pressing operation. The centering of engraved moulds or dies with respect to the perforated disk is then obtained by means of a spindle having a conical bearing surface, which also serves for the fixing of the moulds to the heating plates. During the closing of the plates, alignment is ensured by guide rods or pins. The centering of the spiral with respect to the central hole is obtained to within 150 to 200 .mu.m, which is quite adequate for the contact reading of audio disks. However, such an eccentricity is excessive for the optical reading of disks carrying data recorded on a 1 .mu.m wide track with a pitch of 1.6 .mu.m. In the case of such disks, more precise centering means must be used.
Another method involves carrying out the drilling of the central hole on a compression press with lateral rods but, in view of the precision required, it is not possible to centre the two plates carrying the engraved moulds from the lateral centering members which would be fixed to the periphery of the plates, because expansion phenomena have an asymmetrical character, when a single plate is heated and as a result the problem of centering engraved moulds is not solved.